HD 3167

HD 3167 is a single,^[4] orange-hued star in the zodiac constellation of Pisces that hosts a system with three exoplanets.^[7][8] The star is too faint to be seen with the naked eye, having an apparent visual magnitude of 8.97.^[2] The distance to HD 3167 can be determined from its annual parallax shift of 21.12 mas as measured by the Gaia space observatory,^[1] yielding a range of 154 light years. It has a relatively high proper motion, traversing the celestial sphere at the rate of 0.204″ per year.^[9] Since it was first photographed during the Palomar observatory sky survey in 1953, it had moved over 12.5″ by 2017.^[5] The star is moving away from the Earth with an average heliocentric radial velocity of +19.5 km/s.^[4]

This is an ordinary K-type main-sequence star with a stellar classification of K0 V^[3] and no significant variability.^[2] The star has 86% of the mass of the Sun and 86% of the Sun's radius.^[5] This gives it a bulk density of 5.60+2.15
−1.43 g/cm³.^[5] It is a chromospherically inactive^[4] star and is radiating 56%^[2] of the Sun's luminosity from its photosphere at an effective temperature of 5,261 K. The spin of the star displays a relatively low projected rotational velocity of around 1.7 km/s. It has a near solar metallicity – a term astronomers use for the proportion of elements other than hydrogen and helium in a stellar atmosphere.^[5]

In October 2021, astronomers reported that the orbits of the detected exoplanets hosted by the star are oddly unusual: two planets (HD 3167 c; HD 3167 d) revolve around the star on polar orbits, i.e. orbits that pass over the poles of the star, whereas the third planet (HD 3167 b) orbits around the equator of the star instead.^[10][11]

Planetary system[]

In 2016, data collected during the extended K2 mission of the Kepler space telescope was used to identify two transiting exoplanet candidates orbiting this star, designated HD 3167 b and HD 3167 c. This makes it one of the closest and brightest such multi-transiting stars known. The lack of chromospheric activity makes it ideal for the precise radial velocity (RV) measurements needed to estimate the masses of its planets.^[4] Follow-up RV observations showed additional perturbation signals beyond the two planets already identified.^[12] This led to the discovery in 2017 of a third, non-transiting planet, designated HD 3167 d.^[5]

The close-orbiting body HD 3167 b has a mass of 5.02 M_Earth and radius 1.70 R_Earth. It most likely has had its atmosphere stripped away by the host star, leaving a rocky planet with about 15% iron by mass. HD 3167 b is orbiting HD 3167 with a period of 23.03 hours, an orbital inclination of 83.4°, and an assumed orbital eccentricity of zero – a circular orbit. The semimajor axis of its orbit is 0.01815 AU, or just four times the star's radius.^[5]

The second planet, HD 3167 c, has an orbital period of 29.8454 days and an eccentricity of less than 0.267. The semimajor axis is 0.1795 AU. It has 9.80 M_Earth and 3.01 R_Earth, giving it a low bulk density of 1.97+0.94
−0.59 g/cm³.^[5] This suggests either a mini-Neptune^[12] with a gaseous envelope consisting mainly of hydrogen and helium, or a planet consisting of mostly water. In 2020, transmission spectroscopy measurements of atmosphere has strongly favored the model with high (>700 Solar) metallicity of atmosphere, due to discovered molecular absorption bands which cannot be attributed to hydrogen or helium.^[13] The incident flux from the host star is around 16 times the amount the Earth receives from the Sun, and it is less susceptible to atmospheric stripping than HD 3167 b.

The orbital inclination of HD 3167 d is inclined at least 1.3° away from the orbital planes of the other two exoplanets. Its orbit is expected to remain stable for periods longer than 100 million years only if this inclination is less than 40°. It has an orbital period of 8.509±0.045 d, placing it in between the other two orbits, and shows a minimum mass of 6.90 M_Earth. The true mass is most likely less than Neptune.^[5]